(1) Field of the Invention
The present invention relates to an optical amplifier which amplifies in one batch a wavelength division multiplexed (WDM) signal light incorporating a plurality of optical signals of different wavelengths, as well as to an optical communication system which utilizes the optical amplifier and carries out repeater transmission of the WDM signal light, and in particular relates to a WDM optical amplifier and an optical communication system which display excellent noise characteristics and which will accommodate input light power level over a wide range.
(2) Description of the Related Art
The Wavelength Division Multiplexing (WDM) optical transmission system is a transmission system which, by transmitting a plurality of optical signals of different wavelengths through a single optical fiber, enables an increase in communication capacity. The WDM optical transmission system offers several advantages including low introduction costs due to the fact that existing optical fibers can be utilized, and ease of any future upgrades as the transmission path is bit rate free due to the use of optical amplifiers and the like.
In order to achieve the required transmission characteristics, an important factor for optical amplifiers for use in WDM optical transmission systems is the requirement to maintain the output light at a predetermined constant level while simultaneously suppressing the wavelength dependency of the gain in the signal light band. Specifically, the maintenance at a constant level of the output light power per single wavelength as well as the wavelength flatness of the gain is required even if the input light power varies over a wide range.
An example of an optical amplifier which meets the aforementioned requirements, in which the basic construction thereof comprises the positioning of a variable optical attenuator between the two stages of an optical amplification section of a two stage construction, has been proposed by the present applicants. In the proposed optical amplifier basic construction, automatic gain control (AGC) is carried out at both the former stage optical amplification section and the latter stage optical amplification section to control the gain at a constant level, and automatic level control (ALC) is carried out, by adjusting the amount of optical attenuation at the variable optical attenuator positioned between the two stages, to control the output light level from the optical amplifier at the required constant level. Consequently, even if the power level of the input light varies, the gain wavelength characteristic for each optical amplification section is maintained at a constant level, and moreover the output light level from the optical amplifier is also maintained at the required level.
Optical amplifiers of two stage construction have also been proposed in, for example, Japanese Unexamined Patent Publication No. 8-248455 and Japanese Unexamined Patent Publication No. 6-169122. In the optical amplifiers proposed therein, the gain for the entire optical amplifier is controlled at a constant level, and the wavelength characteristic of the gain is maintained at a constant level even if the input light power changes. Moreover, the applicant of the present invention has also proposed a technique wherein a gain equalizer (optical filter) is used for flattening the gain wavelength characteristic of the optical amplification section (refer to Japanese Patent Application No. 9-216049).
With the aforementioned conventional optical amplifiers, in the case where the input light power is comparatively small, AGC operation of each of the amplification sections is possible, but in the case where the input light power increases and the excitation light power of the former stage optical amplification section reaches an upper limit value, AGC operation of the former optical amplification section stops and the excitation light power is controlled at a constant level, resulting in a reduction in the former stage gain. Consequently, in the case where the excitation light power of the former optical amplification section reaches the upper limit value, in order to keep the gain for the entire optical amplifier at a constant value regardless of the input light power, the gain for the latter optical amplification section is controlled to be increased by an amount equivalent to the gain reduction in the former optical amplification section, thus maintaining the wavelength flatness of the gain at a constant level.
However, with the aforementioned conventional optical amplifiers, in the case where the input light power into the former optical amplification section reaches the upper limit value of the excitation light power, any increase in the input light power will result in the gain wavelength characteristic for each optical amplification section varying from the design value thereof. As a result, in those cases where compensation for the gain wavelength characteristic of the optical amplification section is made based on fixed characteristics referenced to the design value (for example, the use of a gain equalizer with a fixed loss wavelength characteristic in both the former and latter optical amplification sections), the system is unable to cope with variations in the gain wavelength characteristic when the input light power is large, and a situation arises where the signal light power is lost in excessive amounts in the former optical amplification section which has stringent noise characteristics.
Specifically, in conventional optical amplifiers of two stage construction, the gain wavelength characteristics of the former optical amplification section and the latter optical amplification section vary in accordance with the input light power as shown in FIGS. 17(A) and 17(B) respectively. The gain wavelength characteristics shown in FIG. 17 are those where each of the optical amplification sections are known erbium doped optical fiber amplifiers (EDFA) and the wavelength band is the 1.55 xcexcm band (around 1535 nmxcx9c1561 nm).
Focussing on the former optical amplification section, which has a large effect on the noise characteristics of the optical amplifier, as shown in FIG. 17(A), when the input light power is a comparatively small xe2x88x9216.6 dBm/ch the gain at the short wavelength side of the 1.55 xcexcm band is higher than the gain at the long wavelength side of the band. On the other hand, when the input light power increases to xe2x88x929.6 dBm/ch there is insufficient excitation light power to achieve the required gain so that the gain decreases. In such a case the gain at the short wavelength side of the band decreases considerably, to be relatively lower than the gain at the long wavelength side of the band.
Until now, former stage optical amplification sections with gain wavelength characteristics as those described above, were fitted with a gain equalizer with loss wavelength characteristics which were previously designed to correspond with the gain wavelength characteristics for when the input light power was comparatively small (with a relatively large loss at the short wavelength side). Consequently, in the case where the input light power was increased, even though the gain at the short wavelength side of the band decreased, the gain equalizer, which has a fixed loss wavelength characteristic, caused excessive amounts of optical power to be lost at the short wavelength side, generating a problem of inferior noise characteristics for the optical amplifier at the short wavelength side.
FIG. 18 is a diagram which shows the noise characteristics (noise factor) of a conventional optical amplifier as those described above, in accordance with the input light power.
As shown in FIG. 18, when the input light power is comparatively small an approximately uniform noise factor is obtained for the entire width of the 1.55 xcexcm band, but as the input light power increases the noise factor at the short wavelength side of the band becomes relatively greater, meaning the noise characteristics deteriorate for the optical amplifier at the short wavelength side.
It is an object of the present invention to resolve the above issues and provide a WDM optical amplifier and an optical communication system which achieve simultaneously wavelength flatness for both the signal light gain and the noise factor for input light over a wide range of levels, to display excellent noise characteristics.
In order to achieve the above object, a WDM optical amplifier of the present invention equipped with an optical amplification device for amplifying in one batch a WDM signal light, comprises an input light measurement device for measuring input light power, a gain equalization device which is connected to the optical amplification device and has loss wavelength characteristics for suppressing the wavelength dependency characteristics of the gain of the optical amplification device, and moreover is able to vary the loss wavelength characteristics, and a gain equalization control device for controlling the loss wavelength characteristics of the gain equalization device in accordance with the input light power measured by the input light measurement device.
With such a construction, a WDM signal light (input light) input into the WDM optical amplifier is amplified in one batch by the amplification device. At this point, because the optical amplification device has gain wavelength dependency, a gain deviation (tilt) is generated in the WDM signal light following amplification (the output light). As the operating gain of the optical amplification device varies in accordance with the input light power, the gain deviation of the output light will vary dependent upon the input optical level. However, this type of output light gain deviation is suppressed by the gain equalization device with variable loss wavelength characteristics. That is, by using the gain equalization control device to control the variable loss wavelength characteristics of the gain equalization device in accordance with the input light power as measured by the input light measurement device, the gain equalization device is supplied with a loss wavelength characteristic which corresponds with the variation in the gain wavelength characteristic of the optical amplification device, enabling compensation for any gain deviation in the output light. Consequently, wavelength flatness of the gain can be ensured for input light over a wide range of levels.
Furthermore, with the WDM optical amplifier described above, a configuration is also possible where the gain equalization device is provided for each stage of a multi-stage construction optical amplification device, and the gain equalization control device respectively controls the loss wavelength characteristics of each of the gain equalization devices.
With such a construction, even in the case of an optical amplification device of multi-stage construction such as a two stage construction with former and latter optical amplification sections, compensation for the gain deviation generated at each stage is performed by the corresponding gain equalization device.
Moreover, a configuration is also possible where the WDM optical amplifier described above is equipped with a gain constant control device for controlling at a constant level the gain of the optical amplification device, and the gain equalization control device judges whether or not the optical amplification operation of the foremost stage optical amplification device is saturated, based on the input light power measured by the input light measurement device, and then respectively controls the loss wavelength characteristic of each of the gain equalization devices.
With such a construction, because the gain constant control device controls the optical amplification operation of the optical amplification device, ensuring that the gain of the optical amplification device is constant, even if the input light power fluctuates, the gain wavelength characteristics of the optical amplification device will not vary. This gain constant control functions effectively when the optical amplification operation is not saturated, but upon saturation the gain decreases so that the gain wavelength characteristics of the optical amplification device will vary. Consequently, the gain equalization control device judges the saturation of the optical amplification operation based on the input light power, and controls each of the gain equalization devices so that the loss wavelength characteristics correspond with the gain wavelength characteristics at saturation, to thereby obtain output light with flat wavelength characteristics even for high level input light such as that generating saturation of an optical amplification device.
A specific construction of the aforementioned WDM optical amplifier is possible wherein the WDM signal light has a wavelength band of 1.55 xcexcm, the optical amplification device incorporates an erbium doped optical fiber amplifier, and the gain equalization control device controls the loss wavelength characteristics so that when a judgement is made of saturation of the, optical amplification operation in the foremost stage optical amplification device, the amount of loss at the short wavelength side of the 1.55 xcexcm band for the gain equalization device provided at the foremost stage optical amplification device is less than the amount of loss when the optical amplification operation is not saturated.
Furthermore, for WDM optical amplifiers equipped with optical amplification devices of multi-stage construction, it is preferable that of the plurality of gain equalization devices, the gain equalization device provided for the foremost stage optical amplification device is connected to the output side of the foremost stage optical amplification device, and of the plurality of gain equalization devices, the gain equalization device provided for the lattermost stage optical amplification device is connected to the input side of the lattermost stage optical amplification device.
With such a construction, by providing a gain equalization device at the output side of the foremost optical amplification device, any imposing of loss on the WDM signal light input into that optical amplification device can be prevented and the noise characteristics thus improved, and moreover by providing a gain equalization device at the input side of the lattermost optical amplification device, any imposing of loss on the WDM signal light output from that optical amplification device can be prevented and a high efficiency rate ensured for the excitation light power.
Moreover, with the WDM optical amplifier described above, it is preferable that an output level control device is provided for controlling the output light power per single wavelength at a constant level. Specifically, the output level control device may be equipped with a variable optical attenuation section which is connected between the foremost optical amplification device and the lattermost optical amplification device, and an optical attenuation amount control section for controlling the amount of optical attenuation at the variable optical attenuation section so that the output light power per single wavelength attains a constant level.
With such a construction, a WDM signal light is output from the optical amplifier in which the signal light power for each wavelength has been controlled at a predetermined constant value.
In addition, a specific construction of the WDM optical amplifier described above is possible wherein the gain equalization device is equipped with a first optical filter with a fixed loss wavelength characteristic and a second filter with a loss wavelength characteristic which can be varied linearly, and the gain equalization control device controls the loss wavelength characteristic of the second filter in accordance with the input light power measured by the input light measurement device.
Furthermore, with the WDM optical amplifier described above, it is preferable that there is provided an output deviation detection device for detecting, based on spontaneous emission light generated by the optical amplification device, the output deviation between the signal light of each wavelength incorporated in the output light, and the gain equalization control device controls the loss wavelength characteristic of the gain equalization device in accordance with the input light power measured by the input light measurement device and the output deviation detected by the output deviation detection device.
With such a construction, the gain equalization control device also controls the loss wavelength characteristic of the gain equalization device in accordance with the output deviation of the output light, which is detected by the output deviation detection device based on spontaneous emission light. Consequently, output light with wavelength flatness can be achieved even in the case of an input light power with a wavelength characteristic, and furthermore because detection of the output deviation of the output light based on spontaneous emission light enables detection of the deviation of the output light to be conducted regardless of any fluctuation in the number of signal light or the signal light wavelengths, control of the required gain equalization in accordance with the installation environment of the optical amplifier can be carried out with even greater reliability.
Moreover, in another possible construction of the WDM optical amplifier described above, the gain equalization device is equipped with a plurality of gain equalizers which each have a different fixed loss wavelength characteristic, and the gain equalization control device selects one of the plurality of gain equalizers in accordance with the input light power or the like measured by the input light measurement device and connects the selected gain equalizer to the optical amplification device.
With such a construction, by selectively connecting one of a plurality of gain equalizers of a fixed loss wavelength characteristic in accordance with the input light power, a gain equalization device which corresponds to the complex gain wavelength characteristics of the optical amplification device can be achieved comparatively easily.
An optical communication system of the present invention is an optical communication system which is equipped with a plurality of the type of WDM optical amplifiers described above, and which further comprises an optical SN ratio measurement device for measuring the optical SN ratio of the WDM signal light transmitted through the plurality of WDM optical amplifiers, and a gain equalization management device for sending sequentially to the gain equalization control device of each of the plurality of WDM optical amplifiers a management signal for controlling the loss wavelength characteristic of the gain equalization device so that the optical SN ratio measured by the optical SN ratio measurement device is improved beyond a preset value. Moreover, it is preferable that the gain equalization management device sends the management signal preferentially to the gain equalization control device of the WDM optical amplifier located at the transmission end.
With an optical communication system of such a construction, the optical SN ratio of the WDM signal light transmitted through the plurality of WDM optical amplifiers is measured by the optical SN ratio measurement device provided at the reception end, and the loss wavelength characteristic of the gain equalization device of each optical amplifier is then managed by the gain equalization management device so that the measured optical SN ratio is improved beyond a required value. Consequently, even for loss wavelength characteristics resulting from the installation environment of the optical amplifier and variations in the gain wavelength characteristic of the optical amplifier itself, the optimum amount of gain compensation can be applied at the best location within the optical communication system.
Other objects, aspects and benefits of the present invention will become apparent from the following description of embodiments given in conjunction with the appended drawings.